Protists: The big picture

  • All Eukaryotes are protists except plants, animals and fungi
    • mostly unicellular
    • amoeba, slime molds, algae, protozoa


  • Extraordinarily diverse
    • typically larger than prokaryotes
    • functions carried out by organelles
    • membrane bound nucleus


  • First undisputed fossils ~1.8 bya
    • spherical algal protists
    • many protists no hard parts
    • common ancestry unresolved

Basic protist cell: Euglena

How did Eukaryotic cells evolve?

  • Origin of the nuclear membrane?


  • Origin of mitochondria (organelle that produces energy)?
    • All eukaryotic cells have mitochondria
    • so mitochondria are homologous…


  • Origin of chloroplasts?
    • Photosynthetic eukaryotes have chloroplasts
    • some from common ancestor
    • others….?


How did Eukaryotic cells evolve?

  • Origin of the nuclear membrane?


  • Origin of mitochondria (organelle that produces energy)?
    • All eukaryotic cells have mitochondria
    • so mitochondria are homologous…


  • Origin of chloroplasts?
    • Photosynthetic eukaryotes have chloroplasts
    • some from common ancestor
    • others….?


  • Endosymbiosis: Organisms of one species living inside an organism of another species
    • proposed in 1905 but way too crazy

Endosymbiosis Theory: Origin of eukaryotic cells

We do not know whether nucleus or organelles occurred first!

Does data support Endosymbiosis Theory? YES!!!

  • Mitochondria and chloroplasts are about the size of an average bacterium


  • Both organelles replicate independently by fission, as do bacteria
    • have their own ribosomes to make their own proteins
    • mitochondria/chloroplasts each have a circular chromosome


  • Both organelles have double membranes consistent with the engulfing mechanism


  • Genes of each organelle closely resemble ancestors!
    • cyanobacteria for chloroplasts
    • proteobacteria for mitrochondria
    • some genes appeared to have shifted from bacteria to host, possibly explaining why ‘mitochondria’ are not free-living

Red and green algae underwent secondary endosymbiosis




  • Algae ingested by other heterotrophic eukaryotes!
    • led to Euglenoids, Diatoms and Brown algae


  • Chloroplasts transferred to other protists
    • results in chloroplasts not from common ancestor!


  • How do we know this happened?
    • HINT: # of membranes around organelles = ____

Euglena: chloroplast with triple membrane

Eukaryotes have diverse reproduction

  • Haplonic life cycles: most life spent with one set of chromosomes (1N)
    • all cells are haploid (some multi-cellular algae, and fungi)
    • zygote only diploid cell → meiosis into daughter cells


  • Diplonic life cycles: most life with pairs of chromosomes (2N)
    • body cells (e.g., animals) are all diploid
    • sex cells (gametes) are haploid via meiosis → then fuse


  • Asexual life cycles: binary fission
    • nucleus divides, then cell divides into two separate organisms → mitosis
    • many-single celled protists can switch to asexual reproduction


  • There are more….
    • parasites require hosts

Life cycles in Eukaryotes: Alternation of Generations


  • More complex life cycles include an alternation of generations
    • some algae and plants
    • both generations must be multi-cellular


  • AOG = the alternation of multi-cellular haploid and diploid generations
    • diploid (2N) = sporophyte → makes spores
    • haploid (1N) = gametophyte → male/female → make gametes
    • fertilization of gametes makes new sporophyte


  • In some species the different generations look similar while in others they look different

Alternation of Generations: Basic Cycle

Protists matters: Ecology